This application is based on Japanese patent application No. Hei 10-61716 filed on Mar. 12, 1998, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to exposure technique, and more particularly to scan-type reducing projection exposure technique.
In this specification, the term xe2x80x9cexposurexe2x80x9d indicates forming a latent image on an energy sensitive material through the use of an energy beam. The kind of energy beam is not limited, but is typically ultraviolet light.
2. Description of the Related Art
Fining patterns are desired to improve the integration density of a semiconductor IC device. Improving the resolution and making a focusing characteristic uniform in an exposure area are required to fine the patterns. A scan-type exposure technique has been proposed as an exposure method which permits a projection lens system to be small in size.
The scan-type exposure is a technique by which a pattern on a reticle is projected onto a wafer, with both the reticle and the wafer being moved and scanned relative to the projection lens system. For example, when the projection lens system employs the z-axis as its optical axis and projects an object plane lying in a first x-y plane onto an image plane lying in a second x-y plane at a reduced scale of 1/n, the reticle in the object plane is scanned in Y-direction, and the wafer or the image plane is scanned in -Y-direction at a speed of 1/n. Individual points on the reticle are focused on their corresponding positions on the wafer, with the positional relationship between the reticle and the projection lens system being varied.
The focusing characteristic of the projection lens system is uniformalized by varying the position of an object point relative to the projection lens system. Moreover, in case of the exposure of a rectangular area, the shorter sides of the area need only be located in the field of view of the projection lens system.
As in the case of a spherical surface, a surface of a lens used in an exposure system has rotation-symmetrical about the optical axis. However, lenses are normally formed by machining work such as grinding or the like. Machining work often entails machining errors, which make it difficult to attain perfect rotation symmetry.
A lens having a single spherical surface has not only color aberrations, but also various aberrations such as Seidel""s five aberrations. Those aberrations can be improved by using a combination lenses or the like. However, when the lens has machining errors, the occurrence of image deformation due to the machining errors cannot be easily predicted.
FIGS. 6A and 6B are diagrams which schematically illustrate the structure and operation of a scan-type exposure apparatus employing a conventional technique. In FIG. 6A, a projection lens system L1 projects (focuses) a pattern formed on a reticle R1 onto a wafer W1 at a reduced scale of 1/n. When the projection lens system L1 is one which performs the projection on a reduced scale of xc2xc, for example, it projects the pattern formed on the reticle R1 onto the wafer W1, reducing the pattern size to xc2xc.
The reticle R1 is driven along, for example, Y-axis shown in the illustration. The wafer W1 is scanned at a speed of 1/n along -Y-axis which is parallel to the Y-axis but extends in the opposite direction to the Y-axis. In other words, when the scan axis of the reticle is Y, the scan axis of the wafer is -Y/n.
As the reticle is scanned in the Y-direction, points P1a and P2a on the reticle also move parallel to each other, and the image points focused by the system L1 also move parallel to each other. Nevertheless, since the wafer W1 is scanned in the -Y/n-direction in synchronization with the movement of the reticle R1, the positions of image points P1b and P2b on the wafer W1 are unchanged relative to the wafer W1.
The above descriptions have been made assuming that the lens system L1 is perfect. In actual cases, however, the lens system L1 incurs the image deformation due to the machining errors, in addition to the aberrations.
FIG. 6B schematically illustrates in enlarged view the deformation of an image projected by the lens system L1 when a batch exposure is effected in a stationary state at one time. If the lens system L1 was perfect, a rectangular pattern on the reticle R1 would be focused on the wafer W1, with the pattern size simply reduced like a pattern PAX shown by a dotted line in FIG. 6B. If the lens system L1 was such an ideal lens system and the scan exposure was performed using the ideal lens system, the image of the point P1a on the reticle would follow the line P/b on the pattern PAX. The image of the point Pa would be transferred onto the wafer W1, without deviating from the ideal position in a direction perpendicular to the scan axis Y. In fact, however, due to the aberrations and manufacturing errors of the lens system L1, the rectangular pattern is focused as a deformed pattern like a pattern PBX shown by a solid line in FIG. 6B when the exposure is effected in the stationary state. The scan exposure, if performed using such a lens system, will entail the following problems when transferring the pattern onto the wafer W1:
Let us consider the case where the scan axis Y runs vertically in the illustration and the scan axis of the wafer W1 is -Y/n. While the reticle R1 is scanned in the Y-direction, the wafer W1 is scanned in the opposite direction, i.e., the -Y/n-direction. If the image of the point P1a on the reticle scanned (traced) a rectangular pattern S shown by a dotted line, the image point P1b on the wafer W1 would not deviate from the ideal position in the direction perpendicular to the scan direction Y.
However, in the case of a focusing characteristic which projects a pattern like the pattern PBX shown by a solid line, the image of the point P1a traces a line P1b during a scan and deviates by xe2x80x9cd1xe2x80x9d in the horizontal X-direction. Similarly, the image of the point P2a traces a line P2b and deviates by xe2x80x9cd2xe2x80x9d in the X-direction. Only the image point deviation in the direction perpendicular to the scan direction has been explained above. However, the image point deviation occurs also in a direction parallel to the scan direction.
As regards aberration factors of the lens system, it can be predicted and simulated where and to what degree aberrations will occur and have occurred. It can be predicted what kind of aberration averaging will be performed by scanning the reticle and the wafer and to what extent the resultant image will contain the aberrations. The aberrations are rotation-symmetrical about the optical axis, and the extent of the image deformation occurring at the time of the scan exposure depends on the distance from the optical axis.
When the image deformation is due to the machining errors of the lens system, it is difficult to predict where and to what extent the image deformation will occur. When an scan-type reducing projection exposure apparatus has the lens machining errors, it is difficult to predict where in the exposure area and to what degree the image deformation will occur.
In the case where the projection lens system used in the scan-type reducing projection exposure apparatus has machining errors, those errors adversely influence the focusing characteristic of the lens system. However, a technique for reducing the influence of machining errors has not been known conventionally.
An object of the present invention is to provide a scan-type reducing projection exposure method for performing a scan-type exposure so as capable of reducing the influence of the lens machining errors upon the focusing characteristic of the lens system.
Another object of the present invention is to provide a scan-type reducing projection exposure apparatus capable of reducing the influence of the lens manufacturing errors on the exposure characteristics.
According to one aspect of the present invention, there is provided a scan-type reducing projection exposure method comprising: a preparing step of preparing a group of test patterns distributed in an exposure area; a projecting step of projecting the group of test patterns onto a photosensitive member, while changing a scan direction of a scan-type reducing projection exposure apparatus; a developing step of developing the photosensitive member to produce a group of photosensitive patterns; a measuring step of measuring dimensions of a predetermined part of the photosensitive patterns to obtain a distribution of the dimension of the photosensitive patterns in the exposure areas; a determining step of determining a scan direction in accordance with dimension distribution obtained in said measuring step; and an exposing step of performing an exposure using the determined scanning direction.
According to another aspect of the present invention, there is provided a scan-type reducing projection exposure apparatus comprising: a reticle table for mounting a reticle; a wafer table for mounting a wafer; an optical system for projecting patterns on the reticle onto the wafer; a first drive table for moving the reticle table in first and second directions; a second drive table for moving the wafer table in the first and second directions; and arithmetic operation means, to which dimensions of patterns are input, for determining a scan direction which permits that differences between the dimensions of the patterns to be minimum.
The influence of the machining errors of a projection optical system can be reduced by actually projecting and developing the test patterns, while changing the scan direction relative to the lens system, measuring differences between the dimensions of the patterns for each scan direction, and determining the optimum scan direction.
Thus, when the projection optical system used in the scan-type reducing projection exposure technique has machining errors, the influence of the machining errors can be reduced.